A wind turbine blade cross section is typically referred to as a blade profile. The shape of the profile varies with the distance from the blade root. The blade is connected to the hub that is placed in the rotor centre. The profile has a chord, c, and a thickness, t, as shown in FIG. 1. The size of the chord and the thickness as well as the thickness to chord ratio varies as a function of the radius, r, i.e. the distance from the rotor centre to the blade cross section.
In principle, a wind turbine blade consists of a plurality of connected blade profiles. The blade and hence the individual profiles are rotated relative to the rotor plane during operation. The incoming wind is about orthogonal to the rotor plane, but since the blade is in motion, the effective angle and speed of the incoming wind (i.e. corresponding to a steady blade) depend on the speed of rotation of the blade. The effective angle is also referred to as the angle of attack, α, as shown in FIG. 2. The effective wind speed that the profiles see is also referred to as the relative wind speed, w, as shown in FIG. 2.
The response of the aerodynamic profile of the blade to incoming wind may be separated into a lift component orthogonal to the effective incoming wind and a drag component that are in parallel to the effective incoming wind. The size of the components may be expressed as the lift coefficient, CL, and the drag coefficient, CD, respectively, as indicated in FIG. 2. In general, it is desired to have a high lift coefficient and a low drag coefficient.
In pitch regulated wind turbines with variable rotor speed, the variation of the angle of attack, α, due to variation in wind speed is compensated by rotating the individual blades about a longitudinal axis, called pitching, and by controlling the rotor speed. Thereby the average angle of attack may be kept close to a desired value with regard to the average wind speed.
CL-CD Plot
The lift coefficient corresponding to the value at the maximum ratio of CL/CD is referred to as the design lift coefficient, CL,d. The design lift coefficient is found as CL corresponding to the tangent to the CL-CD-curve through (0,0) in a CL-CD plot, as show in FIG. 3. Typically each blade cross section is twisted slightly about the pitch axis so that each profile is operating at an angle of attack that corresponds to the design lift coefficient, CL,d for incoming wind speeds in the interval of 7-11 m/s.
CL-α Plot
In FIG. 4, the lift coefficient CL is plotted as a function of the angle of attack, α. It is observed that CL increases as the angle of attack is increased until αstall, above which the blade begins to stall. The maximum lift coefficient, CL,max, corresponds to the lift coefficient at αstall. The maximum lift coefficient, CL,max varies as a function of the Reynolds number. The Reynolds number is defined as:
  Re  =            w      ·      c        v  Where w is the relative wind speed and ν is the kinematic viscosity of air. The maximum lift coefficient, CL,max, also varies as a function of the roughness of the profile surface particularly on the leading edge. The values of lift coefficients mentioned in the present document refer to profiles with a smooth surface subjected to a two-dimensional airflow.
The angle of attack corresponding to CL,d is referred to as the design angle of attack, αd, and may be identified from a set of a CL-CD plot and a corresponding CL-α plot, as indicated in FIG. 3 and FIG. 4. It is the general perception in the art that a wind turbine should be operated at or near the design lift coefficient, CL,d, to reduce drag and to prevent the blade from stalling accidentally. In other words, CL,operation≈CL,d and αoperation≈αd, where αoperation and CL,operation, respectively, are the mean angle of attack and the corresponding mean lift coefficient during operation
Gusts of Wind
The pitch regulation is not sufficiently fast to respond to individual gusts of wind. Therefore, the instant angle of attack, αgust, upon a gust of wind is shifted to a larger angle of attack than the intended αoperation. Wind turbine blade profiles are therefore traditionally designed such that the design angle of attack, αd, as derived from CLd is substantially lower than αstall. as this prevents the blade from accidental stalling and hence increases the tolerance towards gusts of wind.